1. Field
Example embodiments and methods generally relate to materials and components used in reactors of nuclear power plants.
2. Description of Related Art
Generally, nuclear power plants include a reactor core having fuel arranged therein to produce power by nuclear fission. A common design in U.S. nuclear power plants is to arrange fuel in a plurality of fuel rods bound together as a fuel assembly, or fuel bundle, placed within the reactor core. Power is generated by the nuclear fuel, typically uranium, through a fission chain reaction of the fuel atoms.
Steady-state fission in the reactor fuel releases large amounts of neutrons, which initiate and sustain the fission chain reaction. Conventionally, managing and maintaining a steady-state fission reaction and corresponding power production and safety standards is achieved by managing the amount of neutrons and neutron flux within the core. Managing neutron flux may achieve several goals, including, for example, maximizing power production, equalizing fuel neutron exposure and fissioning or “burn,” minimizing neutron flux peaking, and providing safety margins for safe operation and shut-down of the reactor.
Conventional neutron flux management has several forms. Burnable poisons are one form of neutron flux management conventionally used in nuclear reactors. Burnable poisons typically absorb neutron flux, thereby reducing or “poisoning” fuel reactivity and fission rate, where they are placed. Based on the engineer's knowledge of the reactor core and reactor physics, the engineer can determine areas of the core subject to unwanted amounts of neutron flux at particular points in time during operation and place burnable poisons in those positions. Thus, unwanted flux may be reduced, resulting in a more even and/or safer burn throughout the fuel. Alternatively, burnable poisons may be placed in the core coolant or moderator and reduce reactivity throughout the core, potentially providing easier shut-down of the core and/or reducing reliance on other neutron flux management approaches, such as control rod/blade usage.
Burnable poisons also conventionally have a reduced effect as time passes in an operating core. The more neutrons a particular burnable poison absorbs, the lesser its ability to continue absorbing neutrons. Through this property, burnable poisons may be used to control neutron flux or reactivity at specific time periods subject to unwanted amounts of neutron flux, such as beginning of operating cycles, while having minimal effect at other time periods where the poisoning effect is undesired, such as end of operating cycles.
Conventional burnable poisons include, for example, gadolinium and/or boron compounds. These and related elements have a high absorption cross-section, or probability, for thermal neutron flux commonly found in light water reactors. As the burnable poisons absorb neutrons and lower reactivity, they are converted into other elements with much lower thermal neutron absorption cross-sections, thereby “burning out” over time in the operating core. Gadolinium and/or boron compounds are conventionally fashioned into special rods or fuel additives. In these forms, burnable poisons may be placed at specific axial and radial locations within the core to reduce unwanted levels of neutron flux predicted or experienced at those locations at certain times. Conventional burnable poison elements may be removed from the core and disposed of at the completion of each operating cycle, and new burnable poison elements may be introduced to replace the old, depending on new core characteristics.